190 Mr. De la Beche on the Geology of Jamaica. 



The rocks of the summit of Catherine's Peak, upon which the above rest, 

 seem to be submedial or transition, and are formed of argillaceous slate, lime- 

 stone, and gray sandstones : these dip northerly, as may be seen near Clifton 

 (4228 feet above the sea*) and the northern side of Catherine's Peak. 



Comparison of the Jamaica Rocks with those described hy M. von Humboldt 

 in Mexico and South America which appear to resemble them. 



Transition or Submedial Rocks. — The Jamaica rocks of this class can be 

 said very little to resemble those described as occurring in Quito and Peru by 

 Humboldt in his " Essai sur le Gisement des Roches dans les Deux Hemis- 

 pheres," wherein he states that an immense formation of transition porphyry 

 replaces the grauwacke in those places, and that it is associated with syenite 

 and greenstone. We have seen under the head of trap rocks associated with 

 the Jamaica submedial or transition rocks, that porphyries, syenites, green- 

 stones, &c. are by no means rare on the southern side of the Blue Mountain 

 Range : but these rocks do not replace the grauwacke, on the contrary they 

 are associated with it ; and, taking into consideration the mass of the Jamaica 

 transition rocks, the grauwacke and limestones are much more abundant than 

 the trap. 



The transition rocks of Mexico, which Humboldt has classed under his 

 group 3, and which he composes of transition slate containing grauwacke, 

 greenstone, dark coloured limestones, syenites, and porphyries, would seem 

 more to resemble the Jamaica transition rocks, for the latter might be classed 

 in this division. 



Red-sandstone (gres rouge). The rocks which I have described as red 

 sandstone and conglomerate, coal-measures, and porphyritic conglomerate, 

 porphyries or other trap rocks being associated with the latter, seem analogous 

 to the grhs rouge and porphyre secondaire, which Humboldt describes as oc- 

 curring in several parts of Mexico and South America, and which he classes 

 together as one formation. Commencing with New Spain, he informs us that 

 the schists and transition porphyries of Guanaxuato (platform of Anahuac) are 

 covered by a formation of red sandstone, that it forms the plains of Celaya, 

 Salamanca, and Burras, and that it supports a limestone resembling that of 

 the Jura. He observes that in the older beds {mine de Rayas) he thought he 

 observed a passage from red sandstone into grauwacke : this kind of passage 



* According to my barometrical observations. 



